Porous Sol-gel Films Research Articles

Electrochromic Properties of Sol-Gel Synthesized Macroporous Tungsten Oxide Films Doped with Gold Nanoparticles

Macroporous tungsten oxide thin films were prepared by using a polystyrene microsphere template and the peroxitungstic acid precursor. Gold nanoparticles were introduced into the tungsten oxide film by two different methods, either in the polystyrene template, through a self-assembly process, or they were cast on the surface of the annealed film. Raman imaging experiments showed an enhancement effect of the intensity of the major tungsten oxide bands by the gold nanoislands. The electrochromic properties of gold-doped macroporous tungsten oxide are described and compared to those of the standard porous sol-gel film. The films with gold nanoparticles on the surface of the tungsten oxide film were found to have better electrochromic properties than the standard sol-gel tungsten oxide films and the ones doped with gold in the polystyrene template. The diffusion coefficients of protons in the gold-doped tungsten oxide films were determined from both Cyclic Voltammetry (CV) and Electrical Impedance Spectroscopy (EIS) measurements. The results indicate a faster diffusion of the proton in the porous film, compared with a compact tungsten oxide film. The model that evolves from the EIS data, involves fast charge transfer rates at the electrolyte/tungsten oxide film interface. The results demonstrated an enhancement of the coloration efficiency of the macroporous tungsten oxide films with Au nanoparticles on the surface of the film. The possible involvement of plasmon resonance of Au nanoparticles in the electrochromic process is discussed.

Modification of electrodes with catalytic, size-exclusion films

The application of electroanalytical methodology to the study of liquid-phase samples can be complicated by the adsorption of sample components on the electrode surface. Macromolecules are particularly problematic in this regard. An early means of addressing this problem was to use a membrane permeable to the analyte as a barrier between the sample phase and the electrochemical cell. Amperometric determination of oxygen in biological fluids is a historically important example. This approach was refined by modifying electrodes with semi-permeable, conducting films applied directly to the surface of the working electrode. Cellulose acetate is an example of a conductive material that blocked adsorption of compounds in biological samples but was permeable to analytes such as hydrogen peroxide. Modification of electrodes with ion-exchange films and, more recently, porous sol–gel films was an expansion of this methodology. A complicating factor was that oxidation or reduction of most analytes requires a catalyst. The development of films that are size-exclusion barriers to interferents and incorporate an electron-transfer catalyst is described.

Yttrium disilicate crystallites embedded porous sol-gel thin films for self-cleaning displays

We report on the preparation of self-cleaning and highly transparent yttrium disilicate phosphor host matrix. The yttrium disilicate crystallites were produced in titanosilicate thin films prepared by sol-gel and dip coating method. The thin film phosphor surface was maintained to be superhydrophilic by the synergic effect of the coproduced TiO2 anatase crystals and the high porosity of the film. The high porosity and the tuning of the pore size were attained by the “molecular footprint” technique. It was found that the film with higher contact fraction could retain the superhydrophilicity even in the absence of ultraviolet light. We successfully demonstrate the integration of this property to the films containing yttrium disilicate nanocrystals.

Hybrid sol–gel–conducting polymer synthesised by electrochemical insertion: tailoring the capacitance of polyaniline

The templated growth of polyaniline through porous sol–gel films on ITO substrates has been performed. The polyaniline–silica composites have been synthesized by electrochemical reactive insertion in potentiodynamic and potentiostatic conditions. When the polymer is synthesized by potential step experiments, the shape of the chronoamperograms indicates two regimes for the electrochemical polymerization of PANI. Electropolymerization happens faster within the silica pores than on bare ITO electrodes due to confinement effects of oligomeric species formed upon oxidation. The electrochemical capacitance of the silica–PANI hybrids is several time higher than the capacitance of ITO/PANI synthesized under the same conditions. The silica matrix avoids the electric collapse between vicinal conducting fibres in ITO/silica–PANI but allows the diffusion of ionic species that are in contact with the conjugated polymer, increasing therefore the conducting surface exposed to the electrolytic solution.

Novel hybrid optical sensor materials for in-breath O2analysis

This study focuses on the optimisation and characterisation of novel, ORganically MOdified SILicate (ORMOSIL)-based, hybrid sensor films for use in the detection of O(2) on a breath-by-breath basis in human health monitoring applications. The sensing principle is based on the luminescence quenching of the O(2)-sensitive ruthenium complex [Ru(ii)-tris(4,7-diphenyl-1,10-phenanthroline)], which has been entrapped in a porous sol-gel film. The detection method employed is that of phase fluorometry using blue LED excitation and photodiode detection. Candidate sensor films include those based on the organosilicon precursors, methyltriethoxysilane, ethyltriethoxysilane, n-propyltriethoxysilane and phenyltriethoxysilane. While it has been established previously by the authors that these films exhibit a stable, highly sensitive response to O(2), this study focuses on selecting the material most suited for use in a breath monitor, based on the sensitivity, response time and humidity sensitivity of these films. Key parameters to be optimised include the O(2) sensitivity of the film and the film polarity, i.e. the degree of hydrophobicity. These parameters are directly linked to the precursors used. In this study a n-propyltriethoxysilane-derived O(2) sensor platform was selected as the optimum material for in-breath O(2) analysis due to its short response time, negligible humidity interference and suitable O(2) sensitivity in the relevant range in addition to its compatibility with a single-point calibration strategy.

Application of lactate amperometric sol–gel biosensor to sequential injection determination of l-lactate

This work describes the construction and evaluation of lactate sol–gel biosensors to accomplish the determination of lactate in pharmaceutical products. Lactate oxidase was incorporated in a porous sol–gel film placed onto a platinum-based electrode. Acid and basic catalysis were assessed. When coupled to a sequential injection system (SIA) the biosensor, based on (3-aminopropyl)trimethoxysilane, 2-(3,4-epoxycyclohexyl)ethyl-trimethoxysilane, deionised water, polyethylene glycol 6000 and acid catalyst, presented a range of linearity of 5 × 10 −5 to 5 × 10 −3 M. The analytical usefulness of the developed biosensor was evaluated through analysis of commercial pharmaceutical products containing lactate with a sampling rate of 40 samples h −1. The enzyme remained active for at least 30 days, enabling about 700 determinations without sensitivity decrease.

Nanoelectrode ensembles based on conductive polyaniline/poly(acrylic acid) using porous sol–gel films as template

Porous sol–gel (PSG) film has been utilized as a template for the electrochemical polymerization of aniline in presence of poly(acrylic acid) (PAA). The presence of electroactive polyaniline (PAn)/PAA within the porous skeleton of the sol–gel films has been confirmed using cyclic voltammetry, UV–vis spectrometry and atomic force microscopic measurements. The densities and the sizes of the nanoelectrodes can be controlled easily using electrochemical methods. The conductive polymer “wires” of PAn/PAA formation in PSG matrix can behave as an ensemble of closely-spaced but isolated nanoelectrodes. Moreover, the nanoelectrode ensembles based on conductive PAn/PAA for glucose biosensing are fabricated by immobilization of glucose oxidase (GOx) and Nafion onto the surface of conductive polymer. Owing to the biocompatibility of PSG and electro-activity of PAn/PAA at neutral pH regions, the glucose biosensor shows excellent characteristics and performance, such as low detection limit and fast response time.

Conducting Polymer Growth in Porous Sol−Gel Thin Films: Formation of Nanoelectrode Arrays and Mediated Electron Transfer to Sequestered Macromolecules

We show the templated electrochemical growth of poly(3,4-ethylenedioxythiophene) (PEDOT) into porous sol−gel (PSG) films [50:50 tetraethyl orthosilicate/methyltriethoxysilane (TEOS/MTES)] on indium−tin oxide (ITO) substrates and PEDOT-mediated electron transfer to ferrocene-modified dendrimers encapsulated within these sol−gel matrices. We first describe conditions needed to optimize PEDOT electropolymerization within the porous sol−gel (PSG) films such that barely emergent PEDOT features, ca. 100 nm in diameter, protrude from a PSG thin film surface, functioning as an array of ultramicroelectrodes in diffusion controlled oxidation/reduction reactions of solution probe molecules. We next describe the incorporation of fourth-generation PAMAM dendrimers into these PSG films after (a) conjugation with ferrocenecarboxylic acid (Fc-COOH) to form Fc−PAMAM, (b) conjugation with both Fc-COOH and 3-thiopheneacetic acid (3TAA), or (c) conjugation with both Fc-COOH and 3,4-ethylenedioxythiophenemethanol (2,3-dihydrothieno[3,4-b]-1,4-dioxyn-2-yl or EDTM). Oxidation/reduction of the encapsulated Fc−PAMAM units could be voltammetrically detected after PEDOT growth into the sol−gel film, but with different efficiency, depending upon the composition of the encapsulated dendrimer. Background corrected cyclic voltammograms showed that ca. 0.9% of the ferrocene groups became electrochemically active when Fc−PAMAM alone was incorporated into the PSG film. Up to ca. 20% of these Fc−PAMAM units became electrochemically active, with high rates of electron transfer, when EDTM was conjugated to the Fc−PAMAM dendrimer and when the EDOT monomer was codoped into the sol−gel film. These results argue for the direct, facile electrochemical communication of encapsulated Fc,EDTM−PAMAM units to the ITO substrate via the electrochemically grown PEDOT “wires,” especially when the conducting polymer and the Fc units are held in close proximity on the dendrimer. This is a new approach to control of the redox chemistry of macromolecules encapsulated in inert porous matrices.

Particle size effect on porous Sol–Gel doped cladding U-shaped optical fiber critical micelle concentration (CMC) sensor

In this paper, a sol–gel derived coating U-shaped optical fiber sensors for measurement of critical micelle concentration of surfactants has been studied. The doped sol–gel cladding fiber was used to construct an active cladding U-shaped optical fiber CMC measurement sensor. The porous sol–gel film was deposited a 1000 μm core plastic clad silica fiber after removing the cladding of the fiber at the middle portion nearly 6 cm and bending in U-shaped design. CMC detection is based on an adsorption effect in sample solution of sodium dodecylbenzenesulfonate. Three different types of particles size of sol–gel porous cladding were tested in our lab. Test results show that the sensitivity of the small particle size porous sol–gel cladding U-shaped optical fiber in sensing region is higher than that of large particle size sol–gel cladding based U-shaped.

Enhancing IR Detection Limits for Trace Polar Organics in Aqueous Solutions with Surface-Modified Sol-Gel-Coated ATR Sensors

An experimental investigation into the use of surface-modified sol-gel-coated Si attenuated total reflectance (ATR) mid-infrared sensors was completed to determine the detection limits for trace amounts of polar organic molecules in aqueous solutions. The surfaces of porous sol-gel films were modified to be mostly hydrophobic in order to concentrate the polar organic analytes in the film while largely excluding the water solvent. This modification of the Si ATR sensor coupled with multivariate partial least-squares calibration methods allowed approximately three orders of magnitude improvement in detection limits for acetone when compared to detection limits from univariate analysis of spectra obtained on uncoated Si ATR sensors. The comparable improvement for isopropanol in aqueous solution was over two orders of magnitude. Studies performed on mixtures of acetone and isopropanol in water confirmed that quantitative analyses could be performed on these mixture systems with cross-validated standard errors of prediction (CVSEP) of 0.5 and 8.7 ppm, respectively. Using three times the CVSEP as an estimate of the detection limit translates to detection limits of 1.5 and 26 ppm for acetone and isopropanol, respectively.

Tailoring of Sol−Gel Films for Optical Sensing of Oxygen in Gas and Aqueous Phase

Sol-gel-based optical sensors for both gas-phase and dissolved oxygen have been developed. Both sensors operate on the principle of fluorescence quenching of a ruthenium complex which has been entrapped in a porous sol-gel silica film. A comprehensive investigation was carried out in order to establish optimal film-processing parameters for the two sensing environments. Both tetraethoxysilane and organically modified sol-gel precursors such as methyltriethoxysilane and ethyltriethoxysilane were used. Film hydrophobicity increases as a function of modified precursor content, and this was correlated with enhanced dissolved oxygen (DO) sensor performance. Extending the aliphatic group of the modified precursor further improved DO sensitivity. The influence of water/precursor molar ratio, R, on the sol-gel film microstructure was investigated. R value tailoring of the microstructure and film surface hydrophobicity tailoring were correlated with oxygen diffusion behavior in the films via the Stern-Volmer constants for both gas phase and DO sensing. Excellent performance characteristics were measured for both gas-phase and DO oxygen sensors. The long-term quenching stability of DO sensing films was established over a period of 6 months.

Control of Dopant Adsorption from Aqueous Solution into Nanoporous Sol-Gel Films

The doping of porous sol-gel films by adsorption of cations onto silica from aqueous solutions is demonstrated. The films are fabricated using the sol-gel method, giving nanometer scale porosity, high surface area, and homogeneity on the scale of optical wavelengths. After deposition by spin coating and initial heat treatment, the films are soaked in a salt solution of the desired dopant, and then given a further consolidating heat treatment. Doping with lead, aluminum and calcium has been demonstrated, and consistent and reproducible processes have been established. Results are presented showing control of doping level through pre-doping temperature and surface treatment, doping time and temperature, and doping solution pH. Potential applications in integrated optics are proposed.

Selective-area doping of porous solgel films for integrated optics

A technique for doping of porous films by surface adsorption of ions from aqueous solution is demonstrated. Fabrication of the films by use of the solgel technique gives a nanometer-scale porosity, which provides high doping levels and homogeneity. Doping through a masking layer patterned by photolithography is thus possible. With Pb as a dopant, lead silicate films with refractive indices of 1.46 to 1.55 were obtained, with submicrometer patterning resolution. Linear control of the refractive index is achieved through variation of the solution pH. Applications of this technique in integrated optics and optical elements are proposed.

Growth and characterization of semiconductor nanoparticles in porous sol-gel films

Two methods for the preparation of semiconductor doped sol-gel films, for applications in nonlinear optics, have been studied and compared. In the first, porous films are spun from sols containing the cation precursor, and then reacted with H2S gas, and in the second, the cation is adsorbed onto the pore surfaces of passive films from aqueous solution before the gas reaction. Extensive results for CdS doping are given, and preliminary results are reported for other semiconductor species. It is shown that a sputtered silica layer can seal the structure to allow further heat treatment without loss of dopant. The effects of heat treatment of doped films are described, and the limitation of crystallite growth by pore size is shown.

A distributed optical fibre fluorosensor for pH measurement

A novel distributed optical fibre pH sensor is reported. The principle of operation of the system is based on the detection of time-resolved fluorescence originating at spatially separated sensor sites. Sodium Fluorescein is used as the pH indicator. The fluorophor is immobilised in porous sol-gel films laid over the fibre core. The distributed system has a spatial range of 100 m and a resolution of the order of 1 m. The pH range of the system is observed to be pH 7–11.

Chemical sensors based on hydrophobic porous sol-gel films and ATR-FTIR spectroscopy

A new chemical sensor based on attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy has been fabricated by coating an ATR crystal with a hydrophobic mesoporous silica film. The highly porous coating can extract hydrophobic analytes, such as benzene, from aqueous solutions and concentrate them inside the film. The enrichment of the analytes inside the film provides the sensor with enhanced sensitivity. The hydrophobic porous film can efficiently exclude water from the region probed by the evanescent wave, thus eliminating the spectral interference due to water absorption bands. Measurement of nitrogen adsorption and desorption isotherms of the films using surface acoustic wave techniques has been used to characterize the pore structure of the films. Relationships between the structure of the porous film and sensor performance are discussed.

Dissolved oxygen sensor based on fluorescence quenching of oxygen-sensitive ruthenium complexes immobilized in sol–gel-derived porous silica coatings

A dissolved oxygen sensor based on the quenching of fluorescence from a ruthenium dye complex entrapped in a porous sol–gel film is reported. Sol–gel-derived silica films were fabricated by dip-coating on to planar and optical fibre substrates. The films were pre-doped with the oxygen-sensitive ruthenium complex [RII–tris(4,7-diphenyl-1,10-phenanthroline)], the fluorescence of which is quenched in the presence of oxygen. The structure and behaviour of sol–gel films are dependent on the fabrication parameters. In particular, enhancement of the surface hydrophobicity increases the quenching response in water. This is achieved by using suitable proportions of modified precursors of silica of the form R(OEt)3Si, where R is an alkyl group, in the standard fabrication procedure. It is shown that by increasing the ratio of modified precursor, the quenching response in the aqueous phase increases. A very low limit of detection, 6 ppb, was determined for the modified films. Using a high-brightness blue LED, combined with a miniature photodiode detection system, these results indicate the potential for a low-cost, high-performance, portable dissolved oxygen sensor for use in many varied situations from aeration control to on-line river pollution monitoring.

Characterisation of microporous sol-gel films for optical device applications

This paper describes progress towards a silica-on-silicon integrated optics technology based on sol-gel. In particular, the aim of this work is to use porous sol-gel films as a host for semiconductor microcrystallites, in order to achieve optical Kerr effect devices. Control of crystallite size is important to maximize the nonlinear effect, and also has a strong influence on band-gap, and thus the wavelength of operation. We are examining the control of pore size distribution, via sol-gel process parameters, as a means of setting crystallite size, and for this reason have developed a technique for the measurement of micropore distributions in films. Results of this technique are presented; these give the first detailed measurements of pores below 10 A in diameter. The fabrication and measurement of initial doped films is described, giving strong evidence for quantum confinement.

Evanescent sensing in doped sol-gel glass films

Bromophenol blue, a pH indicator, was entrapped in porous glass films prepared by the sol-gel method. The colorimetric response of the films to pH was assessed visibly and by attenuated total reflectance spectrometry. The entrapped indicator was nonleachable and responded to solution pH changes in a reversible manner. The immobilization of indicator molecules in porous sol-gel films appears to be a particulary promising approach to optical waveguide sensor development.